In recent years, effective use of a water resource has been required for industrial development or increasing population. For that purpose, reuse of discharged water such as industrial discharged water is very important. In order to attain the reuse, it is necessary to purify water, that is, separate other substances from the water. Various methods for separating the other substances from a liquid have been known. Examples thereof include membrane separation, centrifugal separation, activated carbon absorption, ozonization processing, and removal of suspended solids by flocculation. The methods can remove chemical substances significantly affecting environment such as phosphorous and nitrogen contained in the water, and can remove oil and clay or the like dispersed in the water. Of these, the membrane separation is one of methods most generally used to remove insoluble substances in the water. In views of protecting a membrane and increasing the flow rate of water containing a hardly dehydratable substance, a method for using a filter aid is often used. The method is referred to as a precoat method or a body feed method.
On the other hand, as a method for removing fluoride ions from water, a method for precipitating the fluoride using calcium fluoride, a method for adsorbing the fluoride ions using polyaluminum chloride, and a method for recovering the fluorine ions using a polymeric flocculant have been known.
For example, conventionally, a method for mixing precipitated calcium fluoride with a polymeric flocculant to form large flocs, and recovering the flocs has been known. The method disadvantageously results in reduced purity of the calcium fluoride recovered, which complicates the recovery of the calcium fluoride as a valuable resource. The method disadvantageously causes the incorporation of the polymeric flocculant to increase a sludge amount. A method for returning a part of precipitated calcium fluoride to form a crystal nucleus, and recovering large particles having grown from the crystal nucleus has been known. Since the method may not use the polymeric flocculant, the method solves the above-mentioned problem. However, the method disadvantageously returns a part of the calcium fluoride to reduce processing efficiency, and disadvantageously requires a time for growing the calcium fluoride to a crystal having a sufficient size. Furthermore, a method for recovering fluoride ions in water using an aluminum salt (polyaluminum chloride or the like) has been known. However, the aluminum salt is very hardly separated from the water. In this case, it is necessary to use the aluminum salt and the polymeric flocculant in combination.
As described above, the fluorine recovering treatment in the water has a plurality of steps and is troublesome. Examples of the cause include difficult separation caused by a small particle diameter of the calcium fluoride to be produced, and difficult removal of the aluminum salt added into the water.